Heat pump systems for home and commercial buildings comprise a reversible refrigeration system including in indoor coil mounted within the interior of the building being conditioned, and an outdoor coil subjected to ambient air flow. The indoor and outdoor coils trade functions as evaporator and condenser, based on heating or cooling needs for the space to be conditioned. Normally, the heat pump system includes a reversing valve for reversing the flow of refrigerant to and from the compressor relative to the indoor and outdoor coils which are connected in series with the compressor. During cooling mode, the indoor coil becomes the evaporator for the system, and the outdoor coil becomes the condenser. During the heating mode, these coils trade functions, that is, the indoor coil becomes the condenser rejecting heat to the interior of the building, while the outdoor coil becomes the evaporator, picking up heat from the air passing over the outdoor coil. Conventionally, the outdoor coil consists of a number of turns of tubing bearing the refrigerant and normally occupying a horizontal plane. The tubing carries a plurality of closely spaced metal heat exchange fins which project vertically in parallel or side by side fashion. In order to effect the proper heat exchange between the outside air and the refrigerant within the tubing, such outdoor coil assembly further comprises one or more electric motor driven fans which may be either positioned above or below the outdoor coil. The fans operate to force air upwardly through the fins and maximizing heat transfer between the ambient air and the refrigerant within the coil tubing.
When the outdoor functions as an evaporator, particularly at temperatures near freezing, there is a tendency for the moisture within the air stream to condense on the coil tubing and the fins and to be frozen by contact with these heat conductive members which are below freezinfg temperature. The build-up of the frozen condensate causes a restriction in the air flow between the fins, resulting in inefficient heat transfer between the forced air and the refrigerant within the coil itself. This decreased the thermal efficiency of the system. Further, this requires the manufacturer of the outdoor coil to provide fins which are spaced relatively far apart to insure that the presence of the frozen condensate will not totally block air flow over the outdoor coil when the fan motors are energized.
Further, it is necessary to periodically defrost the outdoor coil. Attempts have been made to provide electrical resistance heaters which are mounted to the outdoor coil and, upon energization of the heaters, the generated heat tends to melt the frozen condensate. This normally is achieved at cessation of heat pump operation.
It has been determined that since the refrigerant vapor being discharged from the compressor is at relatively high temperature, by momentary cyclic mode reversal of the heat pump itself, the outdoor coil can be changed from its evaporator function to condenser function, and permit the hot refrigerant vapor discharging from the compressor to achieve defrosting of the coil. Such heat pump systems incorporating reversal of refrigerant or cyclic mode reversal have been fairly successful in achieving the partial removal of the condensate without materially adversely affecting the function of the system in maintaining proper temperature conditions within the environment being conditioned. In most cases, adequate controls are provided to the heat pump or reversible refrigeration system for both reversing the refrigerant flow, causing the outdoor coil to function as a condenser and terminating fan operation so that the melted condensate simply falls downwardly under the influence of gravity from the fins bearing such frozen condensate.
Such control systems are responsive to a signal such as pressure differential for the refrigeration system components in terms of the refrigerant within the closed refrigerant loop including the outdoor and indoor coils as a means for determining the necessity for defrost action, and a further signal indicative of the temperature of the refrigerant within the outdoor coil itself for terminating such defrost. However, in systems to date, the termination of the defrost action usually results in the immediate return of the outdoor coil to its evaporating function with reversal of refrigerant flow within the system, and the energization of the fan motor so as to continue to force the air upwardly over the outdoor coil and particularly the surfaces of the fins. Usually, there remains on the fins some melted condensate which immediately refreezes as a result of the outdoor coil initiating its evaporating function, and at the same time the upward movement of the air flow acts in opposition to gravity and tends to maintain the water droplets on the surface of the fins. Thus, there occurs the refreezing of the condensate in direct opposition to the desired need for complete removal of the condensate.
It is therefore a primary object of the present invention to provide an improved defrost control system for a reversible refrigeration heat pump system which will insure the complete removal of melted condensate prior to the re-initiation of the outdoor coil evaporating function.
It is a further object of the present invention to provide an improved defrost control system for a heat pump or the like in which forced air flow during the termination of the defrost action is provided to assist gravity in the removal of the melted condensate from the fins of the outdoor heat exchange coil.